Calcium current and charge movement of mammalian muscle: action of amyotrophic lateral sclerosis immunoglobulins.

1. The Vaseline‐gap voltage clamp technique was used to record dihydropyridine (DHP)‐sensitive Ca2+ currents (ICa) and charge movement in single cut fibres from the rat extensor digitorum longus (EDL) muscle. Amyotrophic lateral sclerosis (ALS) immunoglobulin G (IgG) action on ICa and charge movement has been characterized. 2. ALS IgG reduced ICa amplitude. The peak ICa of EDL fibres (mean +/‐ S.E.M.) at 0 mV, expressed as amperes per membrane capacitance, was ‐4.79 +/‐ 0.029 A F‐1, while after 30 min incubation in ALS IgG it was ‐2.52 +/‐ 0.04 A F‐1. IgG from healthy patients, and from patients with other diseases (familial ALS, myasthenia gravis, chronic relapsing inflammatory polyneuritis, multiple sclerosis and one sample from Lambert‐Eaton syndrome, LES) did not affect ICa, while IgG from patients with Guillain‐Barré syndrome and one other sample from a patient with LES affected the ICa in a similar way as ALS IgG. 3. The time constant of ICa activation (alpha m) at 0 mV was 44.8 +/‐ 1.4 ms in control, and 36.6 +/‐ 1.5 ms after an incubation of 30 min in ALS IgG. The steady‐state activation curve (m infinity) was shifted to more positive potentials by ALS IgG. 4. The rate constants of activation (range ‐20 to 30 mV) were altered by ALS IgG: alpha m decreased while beta m increased. These data suggest that ALS IgG favours the permanence of the Ca2+ channels in the closed state. 5. The time constant of Ca2+ channels deactivation at ‐90 mV with a pre‐pulse to 0 mV was 4.4 +/‐ 0.5 ms in control and 4.1 +/‐ 0.6 ms in ALS IgG. The relationship between the deactivation time constant and membrane potential was not significantly modified by ALS IgG. 6. ICa inactivation was not affected by ALS IgG. The potentials of half‐inactivation were ‐32.1 and ‐36.6 mV in control and ALS IgG, respectively. Similarly, the rate constants of inactivation (alpha h and beta h) remained unaltered by ALS IgG. 7. We successfully blocked ICa with 100 microM‐TMB‐8 (3,4,5‐trimethoxybenzoic acid 8‐(diethylamino)octyl ester hydrochloride), without major effects on charge movement. We adopted this procedure to study charge movement. ALS IgG reduced charge movement without significant effects on the effective valence and voltage dependence. Qon and Qoff, the charges during and after the pulse, were similarly affected by ALS IgG. 8. The actions of ALS IgG on DHP‐sensitive Ca2+ current and charge movement suggest an interaction between ALS IgG and some component of the DHP‐receptor complex.